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The problem with subdomain 5 is seemingly rather obvious then. It's the combination of moving domain and changing the page URLs at the same time (this often doesn't go well) as well as not having full coverage with the redirects.
I don't know how you should proceed (move or not move), but having moved a page from URL A to URL B, if you now move again you will have double the work to do. The existing A=>B redirect will need to be altered A=>C and a completely new B=>C redirect will need to be created.
You absolutely must not create an A=>B=>C redirection chain.
...it was advised to us to just let go of all those old links and start anew.
When we did introduce those new pages, they popped into the SERPS very quickly, ranked very nicely and then maybe 2 weeks later gone. We knew that would happen though. These quoted results are the first and only thing we have seen since November.
We've run link checkers and everything looks clean, the site is easy to navigate and the sitemap is being downloaded daily and almost every page is fully indexed....